Aerosol Cloud Condensation Nuclei Activity Research Articles

A New Method for Size‐Resolved Aerosol CCN Activity Measurement at Low Supersaturation in Pristine Atmosphere

AbstractUnderstanding aerosol‐cloud interactions under pristine atmospheric conditions is crucial for meaningful global climate change predictions. However, ability of aerosol particles to act as cloud condensation nuclei (CCN) activity of aerosols in pristine atmosphere remains limited, mainly because of the difficulty of measuring it in pristine environments. Given the extremely low CCN number concentration under low supersaturations (SSs) and the complex impact of the kinetic limitation in CCN measurements, size‐resolved CCN activity measurements at low SSs cannot be achieved with traditional methods using differential mobility analyzers and a CCN counter (CCNC) because of the low sample efficiency and the impact of particles with multiple charges. To overcome this difficulty, we propose a new method for measuring size‐resolved CCN activity at low aerosol number concentrations. The new method is based on a combined system comprising an aerodynamic aerosol classifier (AAC) and a CCNC (AAC‐CCNC system). A control program was developed to achieve high‐resolution scans of the SS and particle size within a reasonable time frame. A data inversion scheme, including corrections for the transfer function and kinetic limitation, was developed to obtain accurate size‐resolved CCN activity. In July‐August 2022, the new method was used to measure the size‐resolved CCN activity on the Tibetan Plateau. This new method can advance the size‐resolved CCN measurements, thereby enhancing our understanding of aerosol‐cloud interactions in pristine atmosphere.

Measurement report: High Arctic aerosol hygroscopicity at sub- and supersaturated conditions during spring and summer

Abstract. Aerosol hygroscopic growth and cloud droplet formation influence the radiation transfer budget of the atmosphere and thereby the climate. In the Arctic, these aerosol properties may have a more pronounced effect on the climate compared to the midlatitudes. Hygroscopic growth and cloud condensation nuclei (CCN) concentrations of high Arctic aerosols were measured during two field studies in the spring and summer of 2016. The study site was the Villum Research Station (Villum) at Station Nord in the northeastern region of Greenland. Aerosol hygroscopic growth was measured with a hygroscopic tandem differential mobility analyzer (HTDMA) over a total of 23 d, and CCN concentrations were measured over a period of 95 d. Continuous particle number size distributions were recorded, facilitating calculations of aerosol CCN activation diameters and aerosol κ values. In spring, average CCN concentrations, at supersaturations (SSs) of 0.1 % to 0.3 %, ranged from 53.7 to 85.3 cm−3, with critical activation diameters ranging from 130.2 to 80.2 nm and κCCN ranging from 0.28–0.35. In summer, average CCN concentrations were 20.8 to 47.6 cm−3, while critical activation diameters and κCCN were from 137.1 to 76.7 nm and 0.23–0.35, respectively. Mean particle hygroscopic growth factors ranged from 1.60 to 1.75 at 90 % relative humidity in spring, while values between 1.47 and 1.67 were observed in summer depending on the initial dry size. Although the summer aerosol number size distributions were characterized by frequent new particle formation events, the CCN population at cloud-relevant supersaturations was determined by accumulation-mode aerosols.

Kinetic Limitations Affect Cloud Condensation Nuclei Activity Measurements Under Low Supersaturation

AbstractThe commercial continuous‐flow streamwise thermal‐gradient cloud condensation nuclei (CCN) counter is widely used in aerosol CCN activity measurements which are critical for investigating aerosol‐cloud interactions. In CCN measurements, a critical threshold is needed for distinguishing interstitial aerosols and activated droplets, and a default threshold of 0.75 μm was set in CCN counter. Theoretically, interstitial aerosols could also grow larger than 0.75 μm in CCN counter at low supersaturations, thus theoretical thresholds derived from the Köhler theory were suggested. Here, we report that droplet growth in the CCN counter is kinetically limited and CCN‐active droplets does not reach their theoretical diameters under low supersaturations. Neglecting the kinetic growth limitation in CCN identification would count less CCN and underestimate the aerosol hygroscopicity parameter by up to 50% for supersaturations lower than 0.1%. We recommend that thresholds considering kinetic limitations should be used as new criteria for CCN identification under low supersaturations.

Significantly Enhanced Aerosol CCN Activity and Number Concentrations by Nucleation‐Initiated Haze Events: A Case Study in Urban Beijing

AbstractThe evolution of haze, involving multiple processes such as nucleation, coagulation, and condensation, may exert complex effects on aerosols' cloud condensation nuclei (CCN) activity and number concentration (NCCN). Based on field campaigns carried out in the winters of 2014 and 2016 in Beijing, we show that NCCN was significantly enhanced by the evolution of haze, substantially driven by the nucleation process (or new particle formation). The enhancement factor of NCCN by such nucleation‐initiated haze episodes, E_NCCN, defined as the ratio of NCCN after haze events to NCCN prior to haze events, ranged from 2.2 to 6.5 at a supersaturation (S) = 0.76% and from 4.2 to 17.3 at S = 0.23%, the magnitude of which partially depends on the severity of the haze event. The enhancements are much greater than those previously observed and those from model simulations of contribution from new particle formation. This suggests that CCN sources from new particle formation may be underestimated, needing reevaluation in polluted environments where the subsequent growth of newly formed particles can last 2–3 days, yielding more CCN‐sized particles. We further quantified that the changes in particle size and composition during the nucleation‐initiated evolution of haze are responsible for > 80% and 12–20%, respectively, of the enhancement in CCN activity. The changes in particle composition had a limited impact because most of the ambient particles were already hydrophilic, with hygroscopic parameters of 0.2–0.65.

Influence of pollutants on activity of aerosol cloud condensation nuclei (CCN) during pollution and post-rain periods in Guangzhou, southern China

Atmospheric pollutions have an important impact on aerosol, condensation nuclei (CN) and cloud condensation nuclei (CCN) loadings near the ground through disturbing particle size, number, chemical composition and reactions, mixing state, hygroscopicity, and so on. Aerosols and CCN were measured in urban Guangzhou during pollution and post-rain periods to examine effects of particulate pollutants on aerosol CCN activity and compare their mechanisms between summer and winter. In contrast with different levels of pollutions, particle matter (PM2.5) and number (CN) and CCN almost showed an opposite trend to aerosol activity (CCN/CN). In summer, new particle formation (NPF) events triggered by photochemical reactions (e.g. O3) always occurred in no-pollution daytime, and increased significantly CN and CCN as a dominant contributor to secondary aerosols. Under pollution conditions, the gas-to-particle transition driven by photochemical reactions guided the formation and aging processes of particles in daytime, especially in changing soluble species, whereas atmospheric oxidation and heterogeneous reactions dominated at night. In winter, stagnant weather conditions, high pollutant levels and relatively high RH were in favor of particle growing and aging through enhancing secondary particle formation and heterogeneous reactions. The wet scavenging of precipitation reduced greatly CCN amount by scouring pre-existing particles in winter, and during post-rain period the photochemical reactions did not promote the burst of secondary particle formation in the absence of ozone, compared with summer. The results may provide insights into the relationship between aerosol moisture absorption and pollution that may be useful for improving air quality.

Evidence of a reduction in cloud condensation nuclei activity of water-soluble aerosols caused by biogenic emissions in a cool-temperate forest

Biogenic organic aerosols can affect cloud condensation nuclei (CCN) properties, and subsequently impact climate change. Large uncertainties exist in how the difference in the types of terrestrial biogenic sources and the abundance of organics relative to sulfate affect CCN properties. For the submicron water-soluble aerosols collected for two years in a cool-temperate forest in northern Japan, we show that the hygroscopicity parameter κCCN (0.44 ± 0.07) exhibited a distinct seasonal trend with a minimum in autumn (κCCN = 0.32–0.37); these κCCN values were generally larger than that of ambient particles, including water-insoluble fractions. The temporal variability of κCCN was controlled by the water-soluble organic matter (WSOM)-to-sulfate ratio (R2 > 0.60), where the significant reduction of κCCN in autumn was linked to the increased WSOM/sulfate ratio. Positive matrix factorization analysis indicates that α-pinene-derived secondary organic aerosol (SOA) substantially contributed to the WSOM mass (~75%) in autumn, the majority of which was attributable to emissions from litter/soil microbial activity near the forest floor. These findings suggest that WSOM, most likely α-pinene SOA, originated from the forest floor can significantly suppress the aerosol CCN activity in cool-temperate forests, which have implications for predicting climate effects by changes in biogenic emissions in future.

Comparison of aerosol and cloud condensation nuclei between wet and dry seasons in Guangzhou, southern China

Cloud condensation nuclei (CCN), condensation nuclei (CN) and aerosol chemical composition were measured simultaneously at an urban site of Guangzhou from July to August 2015 and in January 2016, and the seasonal variations of aerosol activated fractions (NCCN/NCN) as well as their relevant influence factors were further studied accordingly. NCN is generally higher in winter (dry season), whereas NCCN and NCCN/NCN are mostly higher in summer (wet season) instead. In particular, NCCN and NCCN/NCN are much lower at smaller supersaturation levels (SS<0.2) in winter. In spite of similar diurnal variations for NCCN and NCN, NCCN/NCN indicates an opposite tendency, relatively lower at midday, dusk and before midnight. Other than the size of particles as well as their chemical composition, some other factors, such as mass, gas precursors, pollutant transportation, meteorological conditions, etc., also contribute to the variations of NCCN and NCCN/NCN. Particles from the local source or local-oceanic combination source cast influence on CN and CCN significantly, while the pollutants originating from and crossing over distant polluted areas contribute largely to CCN/CN. NCN and NCCN are relatively higher under pollution-free conditions in summertime and polluted conditions in wintertime, but NCCN/NCN is just the opposite. On various polluted conditions, aerosol CCN activities are greatly discrepant between summer and winter, especially during mist or heavy haze periods. The results imply that anthropogenic pollutants exert critical impacts on aerosol CCN activation.

Role of organic aerosols in CCN activation and closure over a rural background site in Western Ghats, India

The cloud condensation nuclei (CCN) closure study was performed to exemplify the effect of aerosol chemical composition on the CCN activity of aerosols at Mahabaleshwar, a high altitude background site in the Western Ghats, India. For this, collocated aerosol, CCN, Elemental Carbon (EC), Organic Carbon (OC), sub-micron aerosol chemical speciation for the period from 3rd June to 19th June 2015 was used. The chemical composition of non-refractory particulate matter (<1 μm) as measured by Time of Flight – Aerosol Chemical Speciation Monitor (ToF-ACSM) was dominated by organics with average concentration of 3.81 ± 1.6, 0.32 ± 0.06, 0.15 ± 0.02, 0.13 ± 0.03 and 0.95 ± 0.12 μg m−3 for organics, ammonium, chloride, nitrate and sulphate, respectively. The PM1 number concentration as obtained by Wide Range Aerosol Spectrometer (WRAS) varied from 750 to 6480 cm−3. The average mass concentration of elemental carbon (EC) as measured by OC-EC analyzer was 1.16 ± 0.4 μg m−3. The average CCN concentrations obtained from CCN counter (CCNC) at five super-saturations (SS's) was 118 ± 58 cm−3 (0.1% SS), 873 ± 448 cm−3 (0.31% SS), 1308 ± 603 cm−3 (0.52% SS), 1610 ± 838 cm−3 (0.73% SS) and 1826 ± 985 cm−3 (0.94% SS). The CCN concentrations were predicted using Köhler theory on the basis of measured aerosol particle number size distribution, size independent NR-PM1 chemical composition and calculated hygroscopicity. The CCN closure study was evaluated for 3 scenarios, B-I (all soluble inorganics), B-IO (all soluble organics and inorganics) and B-IOOA (all soluble inorganic and soluble oxygenated organic aerosol, OOA). OOA component was derived from the positive matrix factorization (PMF) analysis of organic aerosol mass spectra. Considering the bulk composition as internal mixture, CCN closure study was underestimated by 16–39% for B-I and overestimated by 47–62% for B-IO. The CCN closure result was appreciably improved for B-IOOA where the knowledge of OOA fraction was introduced and uncertainty reduced to within 8–10%.

Discontinuities in hygroscopic growth below and above water saturation for laboratory surrogates of oligomers in organic atmospheric aerosols

Abstract. Discontinuities in apparent hygroscopicity below and above water saturation have been observed for organic and mixed organic–inorganic aerosol particles in both laboratory studies and in the ambient atmosphere. However, uncertainty remains regarding the factors that contribute to observations of low hygroscopic growth below water saturation but enhanced cloud condensation nuclei (CCN) activity for a given aerosol population. Utilizing laboratory surrogates for oligomers in atmospheric aerosols, we explore the extent to which such discontinuities are influenced by organic component molecular mass and viscosity, non-ideal thermodynamic interactions between aerosol components, and the combination of these factors. Measurements of hygroscopic growth under subsaturated conditions and the CCN activity of aerosols comprised of polyethylene glycol (PEG) with average molecular masses ranging from 200 to 10 000 g mol−1 and mixtures of PEG with ammonium sulfate (AS) were conducted. Experimental results are compared to calculations of hygroscopic growth at thermodynamic equilibrium conducted with the Aerosol Inorganic Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model, and the potential influence of kinetic limitations on observed water uptake was further explored through estimations of water diffusivity in the PEG oligomers. Particle-phase behavior, including the prevalence of liquid–liquid phase separation (LLPS), was also modeled with AIOMFAC. Under subsaturated relative humidity (RH) conditions, we observed little variability in hygroscopic growth across PEG systems with different molecular masses; however, an increase in CCN activity with increasing PEG molecular mass was observed. This effect is most pronounced for PEG–AS mixtures, and, in fact, an enhancement in CCN activity was observed for the PEG10000–AS mixture as compared to pure AS, as evidenced by a 15 % reduction in critical activation diameter at a supersaturation of 0.8 %. We also observed a marked increase in apparent hygroscopicity for mixtures of higher molecular mass PEG and AS under supersaturated conditions as compared to subsaturated hygroscopic growth. AIOMFAC-based predictions and estimations of water diffusivity in PEG suggest that such discontinuities in apparent hygroscopicity above and below water saturation can be attributed, at least in part, to differences in the sensitivity of water uptake behavior to surface tension effects. There is no evidence that kinetic limitations to water uptake due to the presence of viscous aerosol components influenced hygroscopic growth. For the systems that display an enhancement in apparent hygroscopicity above water saturation, LLPS is predicted to persist to high RH. This indicates a miscibility gap and is likely to influence bulk-to-surface partitioning of PEG at high RH, impacting droplet surface tension and CCN activity. This work provides insight into the factors likely to be contributing to discontinuities in aerosol water-uptake behavior below and above water saturation that have been observed previously in the ambient atmosphere.

Characterization and parameterization of aerosol cloud condensation nuclei activation under different pollution conditions.

To better understand the cloud condensation nuclei (CCN) activation capacity of aerosol particles in different pollution conditions, a long-term field experiment was carried out at a regional GAW (Global Atmosphere Watch) station in the Yangtze River Delta area of China. The homogeneity of aerosol particles was the highest in clean weather, with the highest active fraction of all the weather types. For pollution with the same visibility, the residual aerosol particles in higher relative humidity weather conditions were more externally mixed and heterogeneous, with a lower hygroscopic capacity. The hygroscopic capacity (κ) of organic aerosols can be classified into 0.1 and 0.2 in different weather types. The particles at ~150 nm were easily activated in haze weather conditions. For CCN predictions, the bulk chemical composition method was closer to observations at low supersaturations (≤0.1%), whereas when the supersaturation was ≥0.2%, the size-resolved chemical composition method was more accurate. As for the mixing state of the aerosol particles, in haze, heavy haze, and severe haze weather conditions CCN predictions based on the internal mixing assumption were robust, whereas for other weather conditions, predictions based on the external mixing assumption were more accurate.

Optical properties and CCN activity of aerosols in a high‐altitude Himalayan environment: Results from RAWEX‐GVAX

AbstractThe seasonality and mutual dependence of aerosol optical properties and cloud condensation nuclei (CCN) activity under varying meteorological conditions at the high‐altitude Nainital site (~2 km) in the Indo‐Gangetic Plains were examined using nearly year‐round measurements (June 2011 to March 2012) at the Atmospheric Radiation Measurement mobile facility as part of the Regional Aerosol Warming Experiment–Ganges Valley Aerosol Experiment of the Indian Space Research Organization and the U.S. Department of Energy. The results from collocated measurements provided enhanced aerosol scattering and absorption coefficients, CCN concentrations, and total condensation nuclei concentrations during the dry autumn and winter months. The CCN concentration (at a supersaturation of 0.46) was higher during the periods of high aerosol absorption (single scattering albedo (SSA) &lt; 0.80) than during the periods of high aerosol scattering (SSA &gt; 0.85), indicating that the aerosol composition seasonally changes and influences the CCN activity. The monthly mean CCN activation ratio (at a supersaturation of 0.46) was highest (&gt;0.7) in late autumn (November); this finding is attributed to the contribution of biomass‐burning aerosols to CCN formation at high supersaturation conditions.

CCN closure study: Effects of aerosol chemical composition and mixing state

AbstractThis study presents a detailed cloud condensation nuclei (CCN) closure study that investigates the effects of chemical composition (bulk and size resolved) and mixing state (internal and external) on CCN activity of aerosols. Measurements of the chemical composition, aerosol size distribution, total number concentration, and CCN concentration at supersaturation (SS = 0.2–1.0%) were performed during the winter season in Kanpur, India. Among the two cases considered here, better closure results are obtained for case 1 (low total aerosol loading, 49.54 ± 26.42 μg m−3, and high O:C ratio, 0.61 ±0.07) compared to case 2 (high total aerosol loading, 101.05 ± 18.73 μg m−3, and low O:C ratio, 0.42 ±0.06), with a maximum reduction of 3–81% in CCN overprediction for all depleted SS values (0.18–0.60%). Including the assumption that less volatile oxidized organic aerosols represent the soluble organic fraction reduced the overprediction to at most 40% and 129% in the internal and external mixing scenarios, respectively. At higher depleted SS values (0.34–0.60%), size‐resolved chemical composition with an internal mixing state performed well in CCN closure among all organic solubility scenarios. However, at a lower depleted SS value (0.18%), closure is found to be more sensitive to both the chemical composition and mixing state of aerosols. At higher SS values, information on the solubility of organics and size‐resolved chemical composition is required for accurate CCN predictions, whereas at lower SS values, information on the mixing state in addition to the solubility of organics and size‐resolved chemical composition is required. Overall, κtotal values are observed to be independent of the O:C ratio [κtotal= (0.36 ±0.01) × O:C − (0.03 ± 0.01)] in the range of 0.2&lt;O:C&lt;0.81, which indicates that the variation in the chemical composition of aerosols is not well represented by the changes in the O:C ratio alone.

Impacts of new particle formation on aerosol cloud condensation nuclei (CCN) activity in Shanghai: case study

Abstract. New particle formation (NPF) events and their impacts on cloud condensation nuclei (CCN) were investigated using continuous measurements collected in urban Shanghai from 1 to 30 April 2012. During the campaign, NPF occurred in 8 out of the 30 days and enhanced CCN number concentration (NCCN) by a factor of 1.2–1.8, depending on supersaturation (SS). The NPF event on 3 April 2012 was chosen as an example to investigate the NPF influence on CCN activity. In this NPF event, secondary aerosols were produced continuously and increased PM2.5 mass concentration at a rate of 4.33 μg cm−3 h−1, and the growth rate (GR) and formation rate (FR) were on average 5 nm h−1 and 0.36 cm−3 s−1, respectively. The newly formed particles grew quickly from nucleation mode (10–20 nm) into CCN size range. NCCN increased rapidly at SS of 0.4–1.0% but weakly at SS of 0.2%. Correspondingly, aerosol CCN activities (fractions of activated aerosol particles in total aerosols, NCCN/NCN) were significantly enhanced from 0.24–0.60 to 0.30–0.91 at SS of 0.2–1.0% due to the NPF. On the basis of the κ-Köhler theory, aerosol size distributions and chemical composition measured simultaneously were used to predict NCCN. There was a good agreement between the predicted and measured NCCN (R2=0.96, Npredicted/Nmeasured=1.04). This study reveals that NPF exerts large impacts on aerosol particle abundance and size spectra; thus, it significantly promotes NCCN and aerosol CCN activity in this urban environment. The GR of NPF is the key factor controlling the newly formed particles to become CCN at all SS levels, whereas the FR is an effective factor only under high SS (e.g., 1.0%) conditions.

Climate-relevant physical properties of molecular constituents for isoprene-derived secondary organic aerosol material

Abstract. Secondary organic aerosol (SOA) particles, formed from gas-phase biogenic volatile organic compounds (BVOCs), contribute large uncertainties to the radiative forcing that is associated with aerosols in the climate system. Reactive uptake of surface-active organic oxidation products of BVOCs at the gas–aerosol interface can potentially decrease the overall aerosol surface tension and therefore influence their propensity to act as cloud condensation nuclei (CCN). Here, we synthesize and measure some climate-relevant physical properties of SOA particle constituents consisting of the isoprene oxidation products α-, δ-, and cis- and trans-β-IEPOX (isoprene epoxide), as well as syn- and anti-2-methyltetraol. Following viscosity measurements, we use octanol–water partition coefficients to quantify the relative hydrophobicity of the oxidation products while dynamic surface tension measurements indicate that aqueous solutions of α- and trans-β-IEPOX exhibit significant surface tension depression. We hypothesize that the surface activity of these compounds may enhance aerosol CCN activity, and that trans-β-IEPOX may be highly relevant for surface chemistry of aerosol particles relative to other IEPOX isomers.

Surfactants from the gas phase may promote cloud droplet formation

Clouds, a key component of the climate system, form when water vapor condenses upon atmospheric particulates termed cloud condensation nuclei (CCN). Variations in CCN concentrations can profoundly impact cloud properties, with important effects on local and global climate. Organic matter constitutes a significant fraction of tropospheric aerosol mass, and can influence CCN activity by depressing surface tension, contributing solute, and influencing droplet activation kinetics by forming a barrier to water uptake. We present direct evidence that two ubiquitous atmospheric trace gases, methylglyoxal (MG) and acetaldehyde, known to be surface-active, can enhance aerosol CCN activity upon uptake. This effect is demonstrated by exposing acidified ammonium sulfate particles to 250 parts per billion (ppb) or 8 ppb gas-phase MG and/or acetaldehyde in an aerosol reaction chamber for up to 5 h. For the more atmospherically relevant experiments, i.e., the 8-ppb organic precursor concentrations, significant enhancements in CCN activity, up to 7.5% reduction in critical dry diameter for activation, are observed over a timescale of hours, without any detectable limitation in activation kinetics. This reduction in critical diameter enhances the apparent particle hygroscopicity up to 26%, which for ambient aerosol would lead to cloud droplet number concentration increases of 8-10% on average. The observed enhancements exceed what would be expected based on Köhler theory and bulk properties. Therefore, the effect may be attributed to the adsorption of MG and acetaldehyde to the gas-aerosol interface, leading to surface tension depression of the aerosol. We conclude that gas-phase surfactants may enhance CCN activity in the atmosphere.

Aerosol cloud activation in summer and winter at puy-de-Dôme high altitude site in France

Abstract. Cloud condensation nuclei (CCN) size distributions and numbers were measured for the first time at Puy-de-Dôme high altitude (1465 m a.s.l) site in Central France. Majority of the measurements were done at constant supersaturation (SS) of 0.24%, which was also deduced to be representative of the typical in-cloud SS at the site. CCN numbers during summer ranged from about 200 up to 2000 cm−3 and during winter from 50 up to 3000 cm−3. Variability of CCN number was explained by both particle chemistry and size distribution variability. The higher CCN concentrations were measured in continental, in contrast to marine, air masses. Aerosol CCN activity was described with a single hygroscopicity parameter κ. Range of this parameter was 0.29 ± 0.13 in summer and 0.43 ± 0.19 in winter. When calculated using SS of 0.51% during summer, κ of 0.22 ± 0.07 was obtained. The decrease with increasing SS is likely explained by the particle size dependent chemistry with smaller particles containing higher amounts of freshly emitted organic species. Higher κ values during winter were for the most part explained by the observed aged organics (analysed from organic m/z 44 ratio) rather than from aerosol organic to inorganic volume fraction. The obtained κ values also fit well within the range of previously proposed global continental κ of 0.27 ± 0.21. During winter, the smallest κ values and the highest organic fractions were measured in marine air masses. CCN closure using bulk AMS chemistry led to positive bias of 5% and 2% in winter and summer, respectively. This is suspected to stem from size dependent aerosol organic fraction, which is underestimated by using AMS bulk mass composition. Finally, the results were combined with size distributions measured from interstitial and whole air inlets to obtain activated droplet size distributions. Cloud droplet number concentrations were shown to increase with accumulation mode particle number, while the real in-cloud SS correspondingly decreased. These results provide evidence on the effects of aerosol particles on maximum cloud supersaturations. Further work with detailed characterisation of cloud properties is proposed in order to provide more quantitative estimates on aerosol effects on clouds.

Ozone oxidation of oleic acid surface films decreases aerosol cloud condensation nuclei activity

[1] Heterogeneous oxidation of aerosols composed of pure oleic acid (C18H34O2, an unsaturated fatty acid commonly found in continental and marine aerosol) by gas-phase O3 is known to increase aerosol hygroscopicity and activity as cloud condensation nuclei (CCN). Whether this trend is preserved when the oleic acid is internally mixed with other electrolytes is unknown and addressed in this study. We quantify the CCN activity of sodium salt aerosols (NaCl and Na2SO4) internally mixed with sodium oleate (SO) and oleic acid (OA). We find that particles containing roughly one monolayer of SO/OA show similar CCN activity to pure salt particles, whereas a tenfold increase in organic concentration slightly depresses CCN activity. O3 oxidation of these multicomponent aerosols has little effect on the critical diameter for CCN activation for unacidified particles at all conditions studied, and the activation kinetics of the CCN are similar in each case to those of pure salts. SO-containing particles which are acidified to atmospherically relevant pH before analysis in order to form oleic acid, however, show depressed CCN activity upon oxidation. This effect is more pronounced at higher organic concentrations. The behavior after oxidation is consistent with the disappearance of the organic surface film, supported by Kohler Theory Analysis (KTA). The κ-Kohler calculations show a small decrease in hygroscopicity after oxidation. The important implication of this finding is that oxidative aging may not always enhance the hygroscopicity of internally mixed inorganic-organic aerosols.

Oxalate metal complexes in aerosol particles: implications for the hygroscopicity of oxalate-containing particles

Abstract. Atmospheric aerosols have both a direct and an indirect cooling effect that influences the radiative balance at the Earth's surface. It has been estimated that the degree of cooling is large enough to weaken the warming effect of carbon dioxide. Among the cooling factors, secondary organic aerosols (SOA) play an important role in the solar radiation balance in the troposphere as SOA can act as cloud condensation nuclei (CCN) and extend the lifespan of clouds because of their high hygroscopic and water soluble nature. Oxalic acid is an important component of SOA, and is produced via several formation pathways in the atmosphere. However, it is not certain whether oxalic acid exists as free oxalic acid or as metal oxalate complexes in aerosols, although there is a marked difference in their solubility in water and their hygroscopicity. We employed X-ray absorption fine structure spectroscopy to characterize the calcium (Ca) and zinc (Zn) in aerosols collected at Tsukuba in Japan. Size-fractionated aerosol samples were collected for this purpose using an impactor aerosol sampler. It was shown that 10–60% and 20–100% of the total Ca and Zn in the finer particles (&lt;2.1 μm) were present as Ca and Zn oxalate complexes, respectively. Oxalic acid is hygroscopic and can thus increase the CCN activity of aerosol particles, while complexes with various polyvalent metal ions such as Ca and Zn are not hygroscopic, which cannot contribute to the increase of the CCN activity of aerosols. Based on the concentrations of noncomplexed and metal-complexed oxalate species, we found that most of the oxalic acid is present as metal oxalate complexes in the aerosols, suggesting that oxalic acid does not always increase the hygroscopicity of aerosols in the atmosphere. Similar results are expected for other dicarboxylic acids, such as malonic and succinic acids. Thus, it is advisable that the cooling effect of organic aerosols should be estimated by including the information on metal oxalate complexes and metal complexes with other dicarboxylic acids in aerosols.

On the impacts of phytoplankton-derived organic matter on the properties of the primary marine aerosol – Part 2: Composition, hygroscopicity and cloud condensation activity

Abstract. The effect of nanogel colloidal and dissolved organic matter &lt;0.2 μm, secreted by marine biota, on the hygroscopic growth and droplet activation behaviour of the primary marine aerosol was studied. Seawater proxies were prepared by the combination of artificial seawater devoid of marine organics and natural seawater enriched in organic exudate released by laboratory-grown phytoplankton cultures, as described in a companion paper. The primary aerosol was produced by bubble bursting, using a plunging multijet system as an aerosol generator. The aerosol generated from seawater proxies enriched with marine exudate presented organic volume fractions on the order of 8–37%, as derived by applying a simple mixing rule. The hygroscopic growth and cloud condensation nuclei (CCN) activity of the marine organics-enriched particles where 9–17% and 5–24% lower, respectively, than those of the aerosol produced from artificial seawater devoid of exudate. Experiments in a companion paper indicated that the cloud nuclei formation could be enhanced in diatom bloom areas because of the increase in the primary particle production induced by marine organics. The experiments in the present study, however, indicate that the impacts of such an enhancement would be counteracted by the reduction in the CCN activity of the primary particles enriched in marine organics. The extent of the effect of the biogenic matter on the particle behaviour was dependent on the seawater organic concentration and type of algal exudate. Aerosol produced from seawater proxies containing diatomaceous exudate presented higher hydrophobicity and lower CCN activity than those enriched with nanoplankton exudate. The organic fraction of the particles was found to correlate with the seawater organic concentration, without observing saturation of the particle organic mass fraction even for unrealistically high organic matter concentration in seawater. These findings are indicative that discrepancies on the composition of the primary aerosol between different studies could partly be explained by the difference in the nature and concentration of the organic matter in the source seawater employed. Consistently across the experiments, theoretical analysis based on the Köhler model predicted a reduction in the primary marine aerosol CCN activity upon the incorporation of marine organics into the particle composition. This effect is consequence of the replacement of small inorganic sea salt molecules by large molar mass organic molecules, together with a moderate suppression of the surface tension at the point of activation of 5–0.5%, which leads to a dominance of the reduction in the dissolved solute in the Raoult term.